1. Field of the Invention
This invention relates to a circuit board and also to a method of manufacturing a circuit board. More particularly, it relates to a circuit board adapted to connect one or more than one BGA (ball grid array) packages and also to a method of manufacturing such a circuit board.
2. Related Background Art
FIG. 1 of the accompanying drawings schematically illustrates in cross section only part of a multilayer printed circuit board prepared by coating the surfaces of stacked insulators 1 that operate as supports with copper foils 2, which are electric conductors. Referring to FIG. 1, the insulators 1 of the multilayer printed circuit board are typically produced by impregnating woven glass fiber networks with epoxy resin and then hardening them. Note that, in FIG. 1, another copper foil 3 is sandwiched by the insulators 1, or the supports, to operate as internal conductor.
For electrically connecting one of the external copper foils 2 and the internal copper foil 3 of a printed circuit board having a configuration as described above by a known technique, a hole is bored through the internal copper foil 3 from the surface of the printed substrate where the selected external copper foil 2 is located and the wall surface of the hole is plated with copper to form an electroconductive layer 4 as shown in FIG. 2.
However, with such a known technique of boring a hole by means of a drill and forming an electroconductive layer 4, the internal copper foil 3 only minimally contacts the electroconductive layer 4 along the periphery of the hole bored through the internal copper foil 3 so that the electric connection can often be unreliable.
FIG. 3 is a known process proposed to bypass the above identified problem. In FIG. 3, a blind hole is bored, leaving the internal copper foil 3 intact, and an electroconductive layer 4 is formed on the wall surface of the blind hole. Such a blind hole is typically bored by means of a laser beam, using carbon dioxide laser. A blind hole may alternatively be bored in a manner as described below by means of a photolithographic technique. Referring to FIG. 4, photoresist 41 is applied to the surface of the copper foil 2, exposed to light and then photochemically developed for forming a hole 5 through the photoresist 41 by patterning. The exposed portion of the copper foil 2 is then removed by etching to form a hole in the copper foil and, as shown in FIG. 5, a laser beam 6 is made to irradiate the hole 5 bored through the copper foil in alignment with the hole of the photoresist to produce a blind hole through the insulator 1 by removing the glass fiber and the epoxy resin of the insulator 1 and expose the internal copper foil 3.
In the case of a printed circuit board prepared by applying an insulation layer 7 on a copper foil 2 as shown in FIG. 6, a fine hole is bored through the insulation layer 7 and an electric conductor such as solder is inserted (or introduced) into the bored hole to electrically connect the copper foil 2 with a component arranged on the insulation layer 7. Such a fine hole can be produced by etching the insulation layer 7, which is typically made of a photosensitive resin material.
However, with the trend in recent years of extremely exploiting printed circuit boards, even smaller blind holes are required to be formed with a higher degree of precision such that any currently available photolithographic techniques cannot cope with. A proposed alternative technique is the use of a laser beam which is typically a beam of carbon dioxide laser for boring a blind hole. Referring to FIG. 7, showing such a blind hole boring technique, a laser beam 6 is converged and made to directly hit the top insulation layer and produce a blind hole by removing the resin of the insulation layer 7 until the underlying copper foil 2 is exposed.
However, when a blind hole is bored by means of a laser beam of carbon dioxide laser, while the laser beam 6 striking the printed circuit board is reflected by the copper foil 2 or 3 and hence the copper foil 2 under the insulation layer 7 or the internal copper foil 3 remains intact, the resin of the insulation layer 7 or that of the insulator 1 can be carbonized to produce a carbonized layer on the copper foil 2 or on the internal copper foil 3 so that consequently the contact area of the conductors can be reduced and the formation of an electroconductive layer on the wall surface of the bored blind hole, can give rise to a defective plating and/or a poorly reliable electric contact.
Meanwhile, BGA (ball grid array) packages have become commercially available as LSI packages that can meet the requirement of a higher degree of integration and a higher operating speed imposed on LSI devices. FIG. 8 is a schematic cross sectional view of a popular BGA package, which is an OMPAC (over molded pad array carrier) type BGA package. Ball-shaped solder bumps 90 are arranged on the rear surface of substrate 81, while a semiconductor chip 86 is mounted on the front surface of the substrate 81 and connected by wire bonding to the wiring conductors 82, 84 and 85 arranged on the substrate 81 by way of Au wires 87. The semiconductor chip and the wire bonding section arranged on the upper surface of the substrate 81 are molded only at a single side by resin 89. An insulating resin layer 83, which is generally a solder resist layer, is formed between the solder bumps or between the wiring circuits. Such a BGA package is structurally simple and affords a wide ball pitch as solder bumps are arranged on the rear surface of the substrate so that components can be mounted on the substrate without difficulty to allow a high density mounting. Note that the wiring conductors 82, 84 and 85 in FIG. 8 may be used as a signal wire, a power wire and a grounding wire respectively.
On the other hand, FIG. 9 shows a multilayer printed circuit board prepared by a build-up technique and comprising a core substrate 91 made of an organic material. A first circuit pattern 92, a first insulation layer 93, a second circuit pattern 94, a second insulation layer 95, a third circuit pattern 96, a third insulation layer 97 and a fourth circuit pattern 98 are sequentially laid on the core substrate 91 in the above mentioned order. The first circuit pattern and the second circuit pattern on the core substrate are connected with each other by way of a first via hole 100 bored through the first insulation layer and the second circuit pattern and the third circuit pattern are connected with each other by way of a second via hole 101 bored through the second insulation layer, whereas the third circuit pattern and the fourth circuit pattern are connected with each other by way of a third via hole 102 bored through the third insulation layer. The first through third insulation layers are formed by using a photosensitive organic resin material. The first through third via holes are bored by means of a photolithographic technique and selective etching. Subsequently, the lateral walls of the via holes are plated.
BGA package substrates and the multilayer circuit boards as described above are also facing the requirement of reducing the diameter of the openings of the solder resist and that of reducing the diameter of the via holes for connecting layers respectively in order to enhance the wiring density and the wire accommodating capacity of the substrate. In an attempt to meet these requirements, the use of laser beams has also been proposed to replace the popular photovia technique that is based on photolithography. While CO2 laser, excimer laser and UV laser such as third harmonic of YAG are currently used, CO2 laser is most popular as it provides a satisfactorily high processing speed.
However, again, when removing an organic resin insulator material by means of laser, debris of the organic resin material and/or a carbide thereof produced on the circuit patterns of lower layers would not be removed completely and remain as residue. Then, the wall surface of the bored hole would not be perfectly plated under such conditions. Thus, there arises the need of carrying out a desmearing process or a plasma processing operation in order to remove the residues of the organic resin material.
In view of the above described circumstances, the present invention is intended to provide a printed circuit board that is free from the above identified problems and that does not carry any residue of the carbide of resin produced on the surface of the copper foil exposed after boring a blind hole by means of a laser beam.
Accordingly, it is an object of the present invention to provide a highly reliable circuit board that is free from any unnecessary residue left on the surface of one or more than one copper foils thereof that can obstruct electric connection and a method of manufacturing such a substrate.
It is another object of the present invention to provide a circuit board comprising an insulator arranged on a conductor and adapted to be used after exposing the conductor under the insulator by irradiating a laser beam on the insulator, wherein a laser beam absorbing region is provided between the conductor and the insulator.
It is still another object of the present invention to provide a method of manufacturing a circuit board comprising steps of preparing a substrate having an insulator on a conductor, irradiating the insulator with a laser beam and removing the insulator at the area irradiated by the laser beam to expose the surface of the conductor, wherein a laser beam absorbing region is provided between the conductor and the insulator and the laser irradiation step is conducted so as to irradiate at least the laser absorbing region with the laser beam.
Still another object of the present invention is to provide a method of manufacturing a circuit board adapted to be used for connecting one or more than one BGA packages or as a multilayer circuit board prepared by a build-up technique and forming a solder resist pattern or one or more than one viaholes in an insulation layer thereof for interlayer connection by means of laser, wherein the surface of the copper circuit pattern provided under the solder resist or the interlayer insulation layer is subjected to a blackening process.
Still another object of the present invention is to provide a circuit board comprising a copper foil partly removed in advance at spots corresponding to one or more than one given openings for boring blind holes getting to the copper foil under the openings through an insulator by irradiating a laser beam, wherein a thin film apt to be decomposed by absorbing a laser beam is provided between said insulator and said copper foil.
A further object of the present invention is to provide a circuit board comprising a copper foil of circuits having on the surface an insulation layer applied thereto and adapted to produce one or more than one blind holes getting to the copper foil under the insulation layer by irradiating the substrate with a laser beam and partly removing the insulation layer, wherein a thin film apt to be decomposed by absorbing a laser beam is provided on the copper foil of circuits.